The present invention relates to fiber optic transmission systems and more precisely to high bit rate wavelength division multiplex fiber optic transmission systems. In the present context the expression xe2x80x9chigh bit ratexe2x80x9d means a bit rate per channel greater than or equal to 10 Gbit/s.
ITU-T Recommendation G.692, xe2x80x9cOptical interfaces for multichannel systems with optical amplifiersxe2x80x9d, 10/98, appendix II proposes a regular frequency step of 50 GHz. It would be beneficial to be able to conform to this recommendation for high bit rate transmission systems as well; however, a problem that arises in this case is that of coherent crosstalk between adjacent channels. Appendix V of the recommendation proposes spectral intervals that are varied in a pseudo-random manner to reduce four-wave mixing. There is an explicit suggestion to choose unequal spectral intervals for the channel frequencies so that no new optical power generated by four-wave mixing lies in any optical signal channel (see Section V.1.1).
Y. Yano et al., xe2x80x9c2.6 Terabit/s WDM transmission experiment using optical duobinary codingxe2x80x9d, ECOC""96, ThB3.1, pages 5.3 to 5.6, is representative of the best results that can currently be obtained in transmission systems and proposes using duobinary coding for transmission on channels separated by 33.3 GHz. The paper refers to duobinary coding as a technique for reducing bandwidth to achieve good spectral efficiency.
The invention proposes to improve the spectral efficiency in wavelength division multiplex transmission systems and to retain a spectral position of the channels that conforms to the ITU recommendation mentioned above.
To be more precise, the invention proposes a wavelength division multiplex transmission system which has at least one channel for which the spectral interval between adjacent channels on one side is different from the spectral interval between adjacent channels on the other side and which includes a demultiplexing filter attenuating more strongly on the side of the channel in which the spectral interval between adjacent channels is smaller.
For a channel having different spectral intervals between adjacent channels, the higher spectral interval is preferably at least 20% greater than the lower spectral interval.
In a preferred embodiment, the wavelengths of the channels are chosen from a set of wavelengths and the difference between two wavelengths of said set is a multiple of a minimum difference between two wavelengths of said set.
The wavelengths of the channels can also be chosen by eliminating one wavelength in three from a set of regularly spaced wavelengths.
The difference of attenuation by the filter at a frequency offset by half the modulation frequency on either side of the center frequency of the channel is advantageously greater than 5 dB.
The filter can have a symmetrical transfer function with the center frequency of the filter offset relative to the center frequency of the channel. A filter which has an asymmetrical transfer function can also be chosen.